Lithium-ion secondary batteries serve as a power source in portable electronic devices and the like. With the trend of portable electronic devices toward functional diversification and increased power consumption, further enhancement of the capacity of lithium-ion secondary batteries is required. Lithium-ion secondary batteries also serve as power sources in electric tools, electric vehicles, and the like. A lithium-ion secondary battery in an electric vehicle such as battery electric vehicles (BEVs) and hybrid electric vehicles (HEVs) is required to retain excellent charging/discharging cycle characteristics for 10 years or longer, to have a large current load characteristic for driving the high-power motor, and to have high energy density per unit volume so as to enhance the cruising distance.
Carbon materials are classified into carbon materials with a low degree of crystallinity (hereinafter, called amorphous carbon materials) and carbon materials with a high degree of crystallinity (hereinafter, called highly crystalline carbon materials). Either carbon material allows intercalation and deintercalation of lithium and therefore is usable as a negative electrode active material.
Generally, a lithium-ion secondary battery that comprises an amorphous carbon material as a negative electrode active material is known to have high battery capacity and be adaptable to rapid charging/discharging. A lithium-ion secondary battery that comprises an amorphous carbon material, however, is also known that the capacity significantly decreases due to repeated charging/discharging cycles (cycle capacity loss).
On the other hand, a lithium-ion secondary battery that comprises a highly crystalline carbon material as a negative electrode active material is known to have stable cycle characteristics and have lower internal resistance than that of a lithium-ion secondary battery comprising an amorphous carbon material. A lithium-ion secondary battery that comprises a highly crystalline carbon material, however, is not capable of being rapidly charged/discharged. This is because intercalation/deintercalation of lithium ions on the side of the negative electrode active material does not proceed fast enough for rapid charging/discharging and therefore the voltage of the battery rapidly reaches its lower limit or upper limit from which the reaction does not proceed any further.
Various composite materials comprising an amorphous carbon material and a highly crystalline carbon material have been proposed.
For example, Patent Document 1 discloses a negative electrode active material that is produced by heating a mixture of a natural graphite particle and pitch in an inert gas atmosphere at 900 to 1100° C. so as to coat the surface of the natural graphite particle with amorphous carbon.
Patent Document 2 discloses a two-layer carbon material obtained by immersing a highly crystalline carbon material that is to be used as the core in tar or pitch and then drying the resultant or heating the resultant at 900 to 1300° C.
Patent Document 3 discloses a carbon material that is obtained by mixing a graphite particle resulting from granulation of natural graphite or scaly artificial graphite with a carbon precursor such as pitch and then calcining the resulting mixture in an inert gas atmosphere at a temperature ranging from 700 to 2800° C.
Patent Document 4 discloses a composite graphite particle that is obtained by granulating scaly graphite having a d002 of 0.3356 nm, an R value of about 0.07, and an Lc of about 50 nm with mechanical external force applied thereto and then coating the resulting spherical graphite particle with a carbonized material obtained by heating a phenolic resin.
Patent Document 1: JP 2005-285633 A
Patent Document 2: JP 2976299 B
Patent Document 3: JP 3193342 B
Patent Document 4: JP 2004-210634 A